A basic component of a wireless local area network (WLAN) based on an IEEE 802.11 technology is a basic service set (BSS), which consists of stations (STAs) with certain association in a certain area of specific coverage, such as a second basic service set (BSS2) and a third basic service set (BSS3) shown in FIG. 1. In a BSS network, a central station for specially managing the BSS is named as an access point (AP), and other stations (STAs) in the network are associated with the AP. An extended service set (ESS) may be formed by connecting multiple BSS networks with each other through a distribution system (DS). In addition, in the absence of the AP, the stations (STAs) may organize a network by themselves to directly communicate with each other, and this network is an independent basis service set (IBSS), such as a first basic service set (BSS1) shown in FIG. 1. In embodiments of the present disclosure, the access point (AP) and the station (STA) are collectively named as nodes.
Distribution nature of channel access of the WLAN based on the IEEE 802.11 technology brings that a carrier monitoring mechanism is crucial for a collision-free operation. A physical carrier monitoring mechanism logically existing in a PHY layer takes charge of detecting transmission of other nodes. However, due to a reason such as a geographic position, physical carrier monitoring may not detect transmission of all the nodes, namely a hidden node problem exists. For example, as shown in FIG. 2, a data transmission path is established between a first station (STA 1) and an access point (AP), and a signal transmitted from the first station (STA 1) may be detected by the access point (AP) and a second station (STA 2), but for a remote node, a third station (STA 3), only a signal transmitted from the access point (AP) may be detected and a signal of the first station (STA 1) may not be detected. When the first station (STA 1) transmits a signal to the access point (AP), the third station (STA 3) may still think that a channel is in an idle state, thereby causing interference to receiving the signal of the first station (STA 1) by the access point (AP).
A network allocation vector (NAV) is a mechanism which may be used for overcoming the above-mentioned hidden node problem. A function of NAV is located on a MAC layer, and provides a virtual carrier monitoring mechanism to enhance physical carrier monitoring. The mechanism is mainly realized by setting a NAV value in each node. Specifically, when a certain node A transmits a MAC frame to a certain target node, NAV values of all stations (except a target station successfully demodulating the MAC frame) under the coverage of the node A are updated according to a time length field borne in a frame header of the MAC frame. The time length field includes a time value using a microsecond (us) as a unit, and the value may be a continuous time length from the end of the last physical layer convergence procedure (PLCP) protocol data unit (PPDU, PLCP protocol data unit) bearing the MAC frame, in which channel resources are in an occupied state.
In 802.11e, another important concept, namely a transmit opportunity (TXOP) is introduced, which means a bounded time period in which a node may transmit a specific communication category and may be acquired through a competition mechanism of a competition period. Within a TXOP time limit, a certain node exclusively occupies a channel resource and may continuously transmit a data frame, a control frame and a management frame and may receive a MAC response frame. The TXOP has a longest time limit, and when this is ended, a TXOP may be acquired through re-competition. Correspondingly, in enhanced distributed channel access (EDCA), each access category (AC) may also independently acquire the TXOP and exclusively occupy the channel resource. A node which has acquired the TXOP is named as a TXOP holder, and a node correspondingly establishing a transmission link with the TXOP holder is named as a TXOP responder.
After the TXOP holder successfully acquires the TXOP, the TXOP holder and the TXOP responder set NAV values for stations under the coverage of the TXOP holder and the TXOP responder through exchange of a first pair of MAC frames to forbid the stations to access to the channel resource within a NAV time period, unless a station is required by the TXOP holder to become the TXOP responders, and the station feeds back a MAC response frame. In the subsequent TXOP time period, a node setting the NAV continues to monitor a MAC frame it received, and if a destination address of the monitored MAC frame does not match with a local address, a local NAV value is updated according to a field value of a time length field in the MAC frame, and if the destination address matches with the local address, a NAV value stored locally is not updated. In addition, the NAV value in each node may decrease continuously over time, and when the NAV value is 0, a node being set the NAV value may perform competition for channel access through the physical carrier monitoring mechanism. If the TXOP holder discovers that a transmission sequence is null (no service data needs to be transmitted) within the TXOP time limit and it is needed to end the TXOP ahead of time, and the length of the remaining time of the current TXOP exceeds the length of a time for retransmitting a CF_End frame, the TXOP holder transmits the CF_End frame to abandon a right to access to the channel resource, wherein a field value of a time length field in the CF_End frame is 0. The other stations under the coverage of the TXOP holder monitor the CF_End frame, clear the NAV values stored locally according to the field value of the time length field and may perform competition for channel access through the physical carrier monitoring mechanism.
However, the inventor discovers that, the CF_End frame transmitted by the TXOP holder only enables the NAV value of a station under the coverage of the TXOP holder to be updated to 0. A hidden station may not receive the CF_End frame because it is not under the coverage of the TXOP holder, so the NAV value of the hidden station may not change. The hidden station may only continue to wait until the local NAV value decreases to 0 and may enter a channel access competition state. The time of entering the channel access competition state of the hidden station is apparently later than that of a node under the coverage of the TXOP holder. Therefore, the hidden station may not enter the channel access competition state timely according to the situation that the TXOP is ended ahead of time, thus a problem of regional discrimination of a station exists.